Solar energy is an abundant resource that may provide an alternative electrical energy source. However, solar energy can present problems to economically collect, store, and transport. One of the ways to collect and utilize solar energy is through photovoltaic (PV) cells, which convert solar energy directly into electrical energy. This conversion of energy can be provided utilizing i-type (intrinsic), n-type and p-type conductivity regions in semiconductor materials, thereby producing a photo-voltage potential and a photo-current generated when electron-hole pairs are formed in the semiconductor material. These electron-hole pairs are formed as a response to impinging photons in the photovoltaic cell.
The energy absorbed by a semiconductor is dependent on its characteristic band-gap. A semiconductor material's “band-gap energy” is defined as the amount of energy required to free an outer shell electron from its orbit about the nucleus to a free state. In semiconductors, the required energy for an electron to be excited from the valence band to the semiconductor conduction band differs based on the separation between the two bands (i.e., the band-gap). Different material may have different characteristic band-gap energies. Band-gap engineering is the process of controlling or altering the band-gap of a material. Conventional silicon based semiconductor materials used in photovoltaic cells (PVs) have a band-gap energy of about 1.1 eV, i.e., only covering a small portion of the broad range of solar radiation spectrum, which has a useable energy in the photon range of approximately 0.4 eV to 4.0 eV.
Light with energy below the band-gap of the semiconductor will not be absorbed and converted to electrical power. Light with energy above the band-gap will be absorbed, but electron-hole pairs that are created quickly lose their excess energy above the band-gap in the form of heat. Thus, this energy is not available for conversion to electrical power.
Therefore, in order to maximize the absorption and conversion of energy in a photovoltaic cell, multi-layered, multi-junction, or multi-subcell, photovoltaic devices have been developed. These multi-subcell devices utilize various materials having different characteristic band-gap energies so that a wider spectrum of solar energy may be absorbed.
A multi-subcell photovoltaic device conventionally includes multiple layers (i.e., subcells) of semiconductor material in a vertically stacked orientation. Each subcell is designed to absorb and convert a different solar energy or wavelength range than that of another subcell of different material. The subcell first exposed to radiant energy, generally has highest band-gap energy while subcells positioned below it have correspondingly smaller band-gap energies. As a result of this arrangement, energy that is not absorbed at one subcell (i.e., level) may be transmitted and converted in another subcell of the device enabling a broad spectrum of solar energy to be converted into electrical energy.
However, this type of multi junction photovoltaic device is limited by the ability to lattice match a number of dissimilar materials by heteroepitaxial growth. Lattice mismatching between adjacent subcells results in strain and dislocations to form, which reduces the overall efficiency of the photovoltaic device. Typically, materials that may be used in a photovoltaic device are severely limited due to the lattice mismatch problem. Attempts to overcome this problem and increase photovoltaic cell efficiency are described, for example, in U.S. Pat. No. 6,372,980 to Freundlich and U.S. Pat. No. 5,851,310 to Freundlich et al., which disclose photovoltaic cells including one or more quantum wells. In addition, U.S. Pat. No. 6,252,287 to Kurtz et al. discloses multi junction solar cells including an indium gallium arsenide nitride (InGaAsN)/gallium arsenide (GaAs) semiconductor p-n heterojunction to improve energy conversion.
Another area of interest in increasing the efficiency of PV cells relates to the use of quantum dots (QD). The band-gap energy of a quantum dot may be affected, not only by its composition, but also by varying its dimensions. Quantum dots have been incorporated into photovoltaic devices. For example, U.S. patent application Ser. No. 11/038,230, which was filed Jan. 21, 2005 by Fafard (U.S. Publication 2005/0155641 A1, published Jul. 21, 2005), discloses a photovoltaic solar cell including a plurality of subcells, at least one of which includes an epitaxially grown self-assembled quantum dot material. A germanium (Ge) bottom subcell is grown on a germanium substrate with a gallium arsenide middle subcell and a gallium indium phosphorus (GaInP) or aluminum gallium arsenic (AlGaAs) top subcell connected in series. The efficiency of the photovoltaic solar cell is improved by using the self-assembled quantum dot material in the middle subcell instead of bulk gallium arsenide material.
Photovoltaic devices utilize dissimilar materials to obtain the capability to absorb a broad spectrum of solar energies. However, the dissimilar materials create problems of lattice mismatch, which leads to poor crystal quality and limited solar cell efficiency. Accordingly, it would be an improvement in the art to provide a method of forming a photovoltaic device having a number of subcell elements capable of producing a photovoltage at multiple wavelengths of absorbed energy while maintaining a high quality crystal.